This invention relates to a vapor generator and, more particularly, to a sub-critical or super-critical once-through vapor generator for converting water to vapor.
In general, a once-through vapor generator operates to circulate a pressurized fluid, usually water, through a vapor generating section and a superheating section to convert the water to vapor. In these arrangements, the water entering the unit makes a single pass through the circuitry and discharges through the superheating section outlet of the unit as superheated vapor for use in driving a turbine, or the like.
These arrangements provide several improvements over conventional drum-type boilers, and, although some problems arose in connection with early versions of the once-through generators (such as excessive thermal loses, mismatching of steam temperature, the requirement for sophisticated controls and additional valving during startup) these problems have been virtually eliminated in later generation systems.
In these later arrangements, the walls of the furnace section of the generator are formed by a plurality of vertically extending tubes having fins extending outwardly from diametrically opposed portions thereof, with the fins of adjacent tubes being connected together to form a gas-tight structure. During startup, super-critical water is passed through the furnace boundary walls in multiple passes to gradually increase its temperature. This requires the use of headers between the multiple passes to mix out heat unbalances caused by portions of the vertically extending tubes being closer to the burners than others or by the tubes receiving uneven adsorption because of local slag coverage, burners being out of service, and other causes. The use of these intermediate headers, in addition to being expensive, makes it undesirable to operate the furnace at variable pressure because of probability of separation of the vapor and liquid phases within the header and uneven distribution to the down-stream circuit. Therefore, this type of arrangement requires a pressure reducing station interposed between the furnace outlet and the separators to reduce the pressure to predetermined values and, in addition, requires a relatively large number of downcomers to connect the various passes formed by the furnace boundary wall circuitry.
In U.S. Pat. No. 4,116,168, issued on Sept. 26, 1978, and assigned to the same assignee as the present invention, a vapor generator is disclosed which incorporates the features of the system discussed above and yet eliminates the need for intermediate headers, additional downcomers, and a pressure reducing station. These improvements are achieved at least in part by forming a portion of the boundary walls of the furnace section of the vapor generator by a plurality of interconnected tubes extending at an acute angle with respect to a horizontal plane. According to a preferred embodiment of this arrangement, the tubes of the boundary walls defining the upper and lower portions of the furnace section of the vapor generator extend vertically while the tubes in the intermediate furnace portion extend at an acute angle with respect to a horizontal plane. The latter tubes are in fluid flow registry with the tubes in the lower and upper furnace portions, and wrap around the furnace section for at least one revolution.
This use of angularly extending tubes in the intermediate furnace section enables the fluid to average out furnace heat imbalances and be passed through the boundary walls in one complete pass thus eliminating the use of multiple passes and their associated mix headers and downcomers. As a result, the furnace can be operated at variable pressure and the need for a pressure reducing station is eliminated. Also as a result of the angularly extending tubes, a relatively high mass flow rate together with a large tube size is possible when compared to a vertical tube arrangement.
However, although the use of the angularly extending tubes has apparent advantages, there is a problem associated with their use. In particular, several openings must be formed through the aforementioned intermediate furnace portion of one or more of the boundary walls to accomodate burners. Normally, these openings are formed by simply bending the tubes forming the wall in a manner to define the opening(s). However, when the tubes extend at an acute angle with respect to a horizontal plane, it can be appreciated that a casual bending of the tubes to form the openings can result in a portion or portions of the bent tubes extending substantially horizontal. This can create significant problems since if even a relatively short length of any tube extends substantially horizontal or at an angle downwardly from the horizontal, the tube will not drain properly which could result in a severe curtailment in the operating efficiency of the generator.